Gamma ray detectors, based on scintillator, semiconductor, or gaseous materials, can be used to absorb, detect, and localize gamma rays. They are useful in a wide variety of applications ranging from medical imaging to astronomy and homeland security.
One nonlimiting example imaging application is positron emission tomography (PET). In PET and other applications, an event produces a gamma ray photon pair. Using detectors, a line of response (LOR) is determined for the photon pair, and multiple LORs are used to image a subject. Single events may also be used in certain imaging methods by determining an incoming direction. Detectors having good energy and spatial resolution provide a potential for high-resolution image detection systems.
However, in a significant number of events, one annihilation photon deposits all its energy across multiple detector elements. Such events, which are referred to herein as photon multiple interactions events (PMIEs), affect the reliability of current imaging methods. A nonlimiting example PMIE occurs when the photon undergoes inter-crystal scatter (ICS). For example, during a PMIE, depending on the gamma ray energy and the detector material (effective atomic number, density, and thickness), one gamma ray photon might produce a sequence of multiple discrete interactions in the detector, depositing its energy across multiple detector elements, due to Compton scatter and other physical effects. Due to the very short interaction time, it can be difficult to determine the actual interaction sequence.
For a high-resolution detector, a large percentage of the recorded single events and recorded coincidences include PMIEs. However, because it is desirable for an image reconstruction to include all recorded events for high sensitivity, ICS events should be positioned as accurately as possible. Because the earliest interaction defines the correct LOR or incoming direction for an event (subsequent interactions are not aligned with the true LOR or direction because Compton scatter deviates the annihilation photon from its linear trajectory, and an incorrect LOR causes loss of contrast, quantitative accuracy, and spatial resolution), it is useful to know the order of the interactions.